Wind stability of naturally regenerated and planted Douglas-fir stands in coastal Washington, Oregon, and British Columbia

Abstract:

Risk of wind damage is an important factor influencing forest management throughout the world. Managed forest landscapes of the coastal Pacific Northwest are undergoing a transition from dominance by naturally regenerated second-growth stands to Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations. This analysis evaluates the impacts of the managed forest transition on risk of wind damage. Experimental plot data are used to compare tree size variability and stand stability between naturally regenerated and planted stands. In addition, a landscape-scale wind risk rating system is developed to evaluate the impact of stand and management transitions at both stand and larger spatial scales.Naturally regenerated stands tend to develop greater variation of tree sizes compared to plantations. Limited size variation in plantations makes them more susceptible to developing high height to diameter ratios (H/D same units) in the dominant trees. The H/D of a tree is a relative measure of stability under wind and snow loads. H/D can be lowered in plantations through reduced planting densities or early thinning. The higher the initial density the shorter the period during which thinning can effectively lower future H/D values. Thinning requirements in dense plantations make their management inflexible. The flexibility with which a stand can be managed describes the rigidity of intervention requirements and/or potential range of stand development pathways.Shorter rotations in plantations compared to naturally regenerated stands offset much of the increased risk of wind damage caused by higher H/D values. The transition from naturally regenerated to planted stands typically increases the diversity of stand ages within a landscape. Even-aged landscapes have distinct periods of high and low risk, as all stands in the landscape move through susceptibility stages together. Uneven-aged landscapes never reach the same levels of landscape risk; however, some portion of the landscape is typically at high risk. As the transition from naturally regenerated to planted stands continues wind damage may be more common but have a reduced potential for devastating a landscape.